The present invention relates to water soluble photosensitive compositions useful for screen printing emulsions.
The art of screen printing is well known in the printing trade. According to this method of printing, a fine-mesh fabric, is stretched across a frame and an image is formed on the fabric. The screen printing stencil to which the image is attached is then placed against the surface to be imprinted, and ink is forced through the screen onto the surface to print on those areas of the surface not covered by the image.
In screen printing, two main photosensitive stencil systems are used. In the indirect method, a photosensitive composition is coated onto a transparent support, usually a polyester material. The coated support is irradiated with actinic light through transparency after which the image obtained is developed. The wet image on its polymeric backing support sheet is then adhered to a screen and, after drying, the polymeric support is removed, leaving the image on the screen mesh ready for printing.
In the direct method, an image is produced on a screen mesh by photosensitizing a water-soluble colloid with a photosensitizer which can be a polymeric diazonium salt, and then coating the composition onto the screen. The coated layer on the screen, after drying, is exposed to actinic light through a suitable transparency to produce a latent image. On development of the image, the areas irradiated by the actinic light remain as they have become insoluble in the developer, while the areas protected from the actinic light retain the original water-solubility of the water-soluble colloid, are washed away to leave an image which, after drying, is ready for printing.
One such method is disclosed in U.S. Pat. No. 3,246,986. In accordance with this method, a screen is coated with a polymeric emulsion containing a light sensitizer. The coated screen is allowed to dry and then exposed to light through a master such as a photographic negative. The light which passes through the master onto the coated screen causes the polymer to be hardened into a water-insoluble pattern corresponding to that of the master, while those areas of the coated screen protected from the light are unaffected and remain unhardened and water soluble. Thus, in effect, there is created a "latent image" which can be "developed" on the screen by washing it with water to leave the image-defining hardened material on the screen and to form a screen printing stencil. Various polymers and light sensitizers have been used in this method. Typical polymers or colloids disclosed for this purpose are polyvinyl alcohol, partially acrylated polyvinyl alcohol, partially hydrolyzed polyvinyl acetate, partially acetylated polyvinyl alcohol, polyacrylic acid, methyl cellulose and gelatin. Typical light sensitizing compounds have included dichromates and condensation products of diazo diphenylamines with aldehydes. It is known in the art to produce compositions for use in screen printing screens which are diazonium compound-sensitized polyvinyl alcohol/polyvinyl acetate emulsions. Such emulsions have several drawbacks since the photosensitivity is relatively low and contains residual formaldehyde.
Several prior art screen emulsions are known from U.S. Pat. No. 4,118,223; U.S. Pat. No. 3,246,986; U.S. Pat. No. 4,418,138; U.S. Pat. No. 4,362,808; U.S. Pat. No. 3,849,392; and U.S. Pat. No. 4,286,048; as well as U.K. Patent No. 1,312,926 and U.K. Patent No. Application 2,108,986, all of which are incorporated herein by reference.
The present invention provides a new screen printing composition containing a specific admixture of diazonium salts to the effect that a composition is formed having unexpectedly increased photosensitivity while maintaining acceptable solution stability and providing a means for lowering the overall amount of residual formaldehyde in the coating.